An existing device includes a housing with an opening therein which is closed by a lid. The lid includes a frame, and a window which is disposed within and hermetically sealed to the frame, the window being transmissive to radiation in a waveband of interest. The device can be used in a television or a projector to form images, which are typically projected onto some type of screen so that they can be viewed by a person. The device includes within the housing a digital micromirror device (DMD) of a known type. A beam of radiation enters the housing through the window in the lid, is processed by the digital micromirror device to form a plurality of sub-beams which represent an image, and at least some of the sub-beams then exit the housing through the window in order to facilitate the generation of the image, which is projected onto the screen.
This existing lid is made by forming a metal frame which has an opening through it, placing a piece of glass in the opening through the frame, and then heating the frame and glass until the peripheral edges of the glass become fused to the edges of the opening in the frame. The side surfaces of the glass are then ground and polished, and one or more coatings are applied to both sides of the glass. While this existing lid and the process of making it have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
In this regard, different applications require lids of various different sizes, and/or glass windows of various different sizes. Fabricating each lid as a separate part is time-consuming and expensive, due in part to the separate handling and processing needed for each lid, and also due in part to the fact that separate tooling is needed for each different frame size, and the separate tooling is relatively expensive. In this regard, grinding and polishing of the opposite side surfaces of the glass window in each separate frame requires a special support part capable of properly supporting a frame of that size within a double-disk grinding apparatus, and each such support part must be configured to conform to the particular size of the lid. Some lid configurations are not associated with a high-volume market, and high tooling costs can thus represent a significant portion of the overall manufacturing cost of each individual lid.
A different consideration is that, when fusing each piece of glass to the associated frame, impurities in the frame can cause the formation of gases. For example, carbon impurities in the frame can lead to the formation of carbon-based gases. Since the glass is softened by the heat used for fusing, the gases can in turn produce bubbles within the glass. An excessive quantity of bubbles can degrade the optical properties of the glass window to an extent where the lid is considered defective and must be discarded. This obviously reduces the effective yield of the fabrication process. Techniques have been developed to clean metal frames by removing impurities, for example by processing the frames in a disassociated ammonia environment. However, these techniques have not been satisfactory in all respects. In particular, these techniques have helped to reduce the number of impurities and thus the number of gas bubbles in the glass, thereby increasing production yields. But the number of parts which must be discarded as defective is still undesirably high, which in turn causes the cost of the satisfactory lids to be undesirably high.